The distribution of elastic, elaunin and oxytalan fibres in tendons and tendon sheaths was studied by means of light and electron microscopy. The different fibres were characterised by comparing their typical ultrastructural pictures with a combination of selective staining techniques for light microscopy with the appropriate enzymatic digestions. Since it was observed that the rat tail tendon and its associated sheaths are not homogeneous structures from the point of view of the distribution of the elastic system fibres, a systematic description of the differential distribution of the oxytalan, elaunin and elastic fibres in the tendon and in the endo-, peri-, epi- and paratendineum is presented. Mature elastic fibres are present in the tendon and in the para-, epi- and endotendineum, whereas these fibres could not be detected in the peritendineum. Very many elaunin fibres are characteristically recognised in the endotendineum; these fibres are present in the peritendineum and in the tendon fascicles too, while the epi- and paratendineum are devoid of elaunin fibres. The peri- and endotendineum contain numerous oxytalan fibres, whereas these fibres are absent from the tendon fascicles and the epi- and paratendineum. The functional implications of the foregoing findings are discussed.